In this paper a description is given of the SFXC software correlator, developed and maintained at the Joint Institute for VLBI in Europe (JIVE). The software is designed to run on generic Linux-based computing clusters. The correlation algorithm is explained in detail, as are some of the novel modes that software correlation has enabled, such as wide-field VLBI imaging through the use of multiple phase centres and pulsar gating and binning. This is followed by an overview of the software architecture. Finally, the performance of the correlator as a function of number of CPU cores, telescopes and spectral channels is shown.

While studying extraplanar neutral hydrogen in the disk-halo transition of the inner Galaxy, we have discovered what appears to be a huge superbubble centered around l % 30 , whose top extends to latitudes >25 at a distance of about 7 kpc. It is detected in both H i and H. Using GBT, we have measured more than 220,000 H i spectra at 9 0 angular resolution in and around this structure. The total H i mass in the system is %10 6 M , and it has an equal mass in H + . The plume of H i capping its top is 1:2 ; 0:6 kpc in l and b and contains 3 ; 10 4 M of H i. Despite its location (the main section is 3.4 kpc above the Galactic plane), the kinematics of the plume appears to be dominated by Galactic rotation, but with a lag of 27 km s À1 from corotation. At the base of this structure there are ''whiskers'' of H i several hundreds of parsecs wide, reaching more than 1 kpc into the halo; they have a vertical density structure suggesting that they are the bubble walls and have been created by sideways rather than upward motion. They resemble the vertical dust lanes seen in NGC 891. From a Kompaneets model of an expanding bubble, we estimate that the age of this system is %30 Myr and its total energy content $10 53 ergs. It may just now be at the stage where its expansion has ceased and the shell is beginning to undergo significant instabilities. This system offers an unprecedented opportunity to study a number of important phenomena at close range, including superbubble evolution, turbulence in an H i shell, and the magnitude of the ionizing flux above the Galactic disk.

The class of type Ic supernovae have drawn increasing attention since 1998 owing to their sparse association (only four so far) with long duration gamma-ray bursts (GRBs). Although both phenomena originate from the core collapse of a massive star, supernovae emit mostly at optical wavelengths, whereas GRBs emit mostly in soft gamma-rays or hard X-rays. Though the GRB central engine generates ultra-relativistic jets, which beam the early emission into a narrow cone, no relativistic outflows have hitherto been found in type Ib/c supernovae explosions, despite theoretical expectations and searches. Here we report radio (interferometric) observations that reveal a mildly relativistic expansion in a nearby type Ic supernova, SN 2007gr. Using two observational epochs 60 days apart, we detect expansion of the source and establish a conservative lower limit for the average apparent expansion velocity of 0.6c. Independently, a second mildly relativistic supernova has been reported. Contrary to the radio data, optical observations of SN 2007gr indicate a typical type Ic supernova with ejecta velocities approximately 6,000 km s(-1), much lower than in GRB-associated supernovae. We conclude that in SN 2007gr a small fraction of the ejecta produced a low-energy mildly relativistic bipolar radio jet, while the bulk of the ejecta were slower and, as shown by optical spectropolarimetry, mildly aspherical.

HESS J1943+213, a TeV point source close to the Galactic plane recently discovered by the H.E.S.S. Collaboration, was proposed to be an extreme BL Lacertae object, though a pulsar wind nebula (PWN) nature could not be completely discarded. To investigate its nature, we performed high-resolution radio observations with the European Very Long Baseline Interferometry Network (EVN) and reanalyzed archival continuum and H i data. The EVN observations revealed a compact radio counterpart of the TeV source. The low brightness temperature and the resolved nature of the radio source are indications against the beamed BL Lacertae hypothesis. The radio/X-ray source appears immersed in a ∼1 elliptical feature, suggesting a possible galactic origin (PWN nature) for the HESS source. We found that HESS J1943+213 is located in the interior of a ∼1 • diameter H i feature and explored the possibility of them being physically related.

No abstract